Embodiments of the invention generally relate to power converters and more particularly to multilevel power converters.
There is a growing need to deliver power over long distances to remote locations which often requires transmission systems which are based on high voltage power transmission due to transmission efficiency and power transfer capability concerns. Historically alternating current (AC) transmission systems have been used for high voltage power transmission, however, the AC transmission systems suffer from undesired cable capacitance that results in charging current requirements. In contrast, high voltage direct current (HVDC) transmission provides a more efficient way to transmit high voltage power over long distances.
In an HVDC transmission system, power converters are often used to convert AC power to DC power at the transmitting substation and to convert the transmitted DC power back to AC power at the receiving substation. In one approach, these power converters have a modular multilevel structure where each phase has a stacked arrangement of modules.
Each of the modules comprises an AC-DC power converter that converts a fraction of the AC power to the DC power at the transmitting substation. In one approach, the modular AC-DC power converters include voltage source power converters which are coupled to each other to form a modular stacked multilevel power converter that supports a unidirectional DC current flow. However, presently two level and three level voltage source modular power converters are used which results in a high number of modules being coupled together to form the modular stacked power converter with a desired AC-DC power conversion capability. Higher numbers of modules results in complexity and high costs.
Hence, there is a need for an improved system to address the aforementioned issues.